\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{circuitikz}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes}
\ctikzset{bipoles/length=1cm}
\begin{document}
\small
\begin{circuitikz}[>=latex, scale=1,european]
  % \useasboundingbox(0.9,0)rectangle(5.1,5);
    \ctikzset{inductors/scale=0.75, inductors/width=2.5, inductors/coils=8} \draw (0,0)--(2,0) to [american inductor] (2,2.8)--(0,2.8);
\draw (2,0)--(4,0);
\draw (4,1.4)--(2,1.4);
\draw [<->](0,.1)--node[fill=white]{$U_1$}(0,2.8-.1);
\draw [<->](4,.1)--node[fill=white]{$U_2$}(4,1.4-.1);
\draw [ultra thick](1.7,.1)--node[left]{$n_1$} (1.7,2.7-.1);
\draw (0,0) [fill=white] circle (1.5pt);
\draw (0,2.8) [fill=white] circle (1.5pt);
\draw (4,0) [fill=white] circle (1.5pt);
\draw (4,1.4) [fill=white] circle (1.5pt);
\node at (2.3, .7){$n_2$};
\end{circuitikz}
\end{document}